Why are you looking at 100FX instead of gigabit? The availability of
gigabit gear is much better, and I'd expect it to end up cheaper.

Because a 8-port Fiber GigaBit Switch cost endless!

We're talking about one fiber run, right? So something like
https://www.newegg.com/Product/Product.aspx?Item=12K-008X-00023 would
suffice, right? How much are you getting the 100fx gear for? If you're
looking at used 100fx equipment then an apples to apples comparison of
used gigabit gear brings the cost down into the $50 range. Add a bit
more for a couple of SFPs, but I still don't understand where the cheap
100FX equipment comes from. The other low cost option would be a pair of
100TX to 100FX media converters.